An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. A printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the nozzle, or a piezoelectric device that changes the wall geometry of the ink pressurization chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the recording medium is moved relative to the printhead.
A common type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a recording medium advance direction by rotating a feed roller and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the recording medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
Conventionally the position of the carriage along the carriage scan direction is monitored by a linear encoder, and the amount of rotation of the feed roller is monitored by a rotary encoder. Such monitoring of the carriage and the feed roller is used by the printer controller to control the firing of droplets from the array of drop ejectors, and to control the amount of feed roller rotation such that the desired image is printed on the recording medium. As is known in the art, sources of error can be introduced in the recording medium position after feed roller rotation, due for example to feed roller diameter errors, feed roller eccentricity, or recording medium slippage relative to the roller.
It is desired to accurately track the position of the carriage and the amount of recording medium advance with fewer sensors. U.S. Pat. No. 7,275,799 (hereinafter referred to as '799) discloses the use of a carriage-mounted, two-dimensional sensor to track both carriage position and paper feed amount by illuminating the paper with coherent light (for example from a semiconductor laser), monitoring the motion of a speckle pattern (interference pattern) with the two-dimensional sensor, and multiplying by a predetermined coefficient. A limitation however, is that for printing of some documents, such as borderless photographs, the illuminated region goes off the paper on at least one side of the paper as the carriage is scanned back and forth during printing. In some cases the surface of the platen can be used to generate a speckle pattern so that carriage motion can still be monitored, even if the illumination region is no longer on the paper. If the paper is not in the region of illumination, however, '799 only provides for controlling the amount of paper feed using the average of previous feed amounts.
The monitoring of paper feed by tracking the motion of a speckle pattern from an idle roller is disclosed in U.S. Pat. No. 7,147,316 (hereinafter referred to as '316). In this approach, the idle roller (as referred to in '316) is in contact with the paper being fed. A surface of the roller is illuminated by a laser and the motion of the speckle pattern of the rotating idle roller is detected by a two-dimensional sensor, where both the laser and the two-dimensional sensor are mounted in fixed position relative to the roller. In other words, they are not carriage mounted. Thus, the idle roller remains illuminated for back and forth carriage passes. With a carriage mounted laser and two-dimensional sensor as disclosed in '799, as well as a stationary mounted laser and two-dimensional sensor as disclosed in '316 both carriage position and paper feed amount can be tracked even for borderless printing (at least until the trail edge of the paper is no longer in contact with the idle roller).
Competitive inkjet printer market pressures require functionality at lower cost. What is needed is an apparatus that can monitor the carriage position and the recording medium feed amount with a single sensor even for borderless printing. If the single sensor could also be used to inspect print test patterns, that would provide further advantages.